PSA processes are normally operated by alternating adsorption and desorption of certain gaseous components using a bed of a suitable adsorbent material. When a vessel containing an adsorber bed is pressurized with a gaseous feed mixture, certain components of the feed gas are adsorbed on the bed material and are retained when the residual gas mixture is withdrawn as product. The withdrawn product mixture is rich in one or more selected gases, depending upon the feed mixture and the properties of the bed material. Next, the bed is regenerated, usually by venting to the environment followed by purging with a purging gas, so as to desorb the retained components and prepare the bed for another adsorption-desorption cycle.
The yield of a PSA process is defined as the ratio of the quantity of selected gas in the PSA product to that in the feed. The productivity of the adsorbent material, called the sieve specific product, is the quantity of gas produced per unit time divided by the quantity of adsorbent material.
In a conventional PSA process, the bed is typically purged with a portion of the gas that has been previously withdrawn as product. Thus, the impurity levels in the bed cannot be reduced below that of the PSA product itself. This constitutes a limit on the extent to which the bed is regenerated and, thus, on the yield and the productivity of the adsorbent material.
UK patent no. 1,586,961 to BOC Limited discloses an oxygen PSA process in which the PSA product is cryogenically separated to produce higher purity oxygen and a nitrogen waste gas. The nitrogen waste gas, which has a lower oxygen concentration than the PSA product, is used to purge the adsorbent beds.